It is often necessary to quickly, accurately and inexpensively measure neutron spectra in low earth orbits covering several energy ranges. High-energy cosmic rays produce neutrons in the upper atmosphere are a particular concern because such neutrons pose a threat to airborne semiconductor equipment such as the memory devices in flight control equipment. Neutrons threaten these devices by causing bit-flips leading to failures in the flight control and navigational equipment, and thereby endangering the operation of both high-flying aircraft like the Concorde and lower altitude commercial aircraft.
There has been a long-standing need to characterize neutron spectra so that physicists and equipment designers can better predict aircraft upset rates and design systems to avoid catastrophic aircraft failures. The general operating principle for neutron spectrometers is that neutrons interact with certain atoms to produce recoil protons that travel in relatively straight lines, as described in Kronenberg, S. and H. Murphy, “Energy Spectrum of Protons Emitted From a Fast-Neutron-Irradiated Hydrogenous Material”, Radiation Research 12, 728-735 1960.
Several types of detectors that have been used in prior art neutron spectrometers of this type to measure the recoil protons. One of the earliest applications described in Kronenberg, S., “Fast Neutron Spectroscope for Measurements in a High Intensity Time Dependent Neutron Environment”, International Symposium on Nuclear Electronics”, Paris France, Comptes Rendus, May 1964. That device utilized a scintillation counter, consisting of cesium iodide and a photomultiplier and solid state devices. A variation of that approach employing a PMOS transistor was described in Kronenberg, S. and G. J. Brucker, “The Use of Hydrogenous Material for Sensitizing PMOS Dosimeters to Neutrons”, IEEE Trans. Nucl. Sci. Vol. 42, No. 1, February 1995.
One significant limitation of these prior art devices is that they can only count protons and can neither characterize neutron spectra nor generate the original neutron spectra. These prior art neutron spectrometers suffered from a number of other disadvantages, limitations and shortcomings because of their size, weight cost and complex circuitry, making them unsuitable for use in spacecraft and other airborne applications. In fact, the NASA Goddard Space Flight Center recently requested proposals for the measurement of high-energy spectra with a spectrometer on-board a satellite or the Shuttle spacecraft.
To overcome the prior art's inability to characterize neutron spectra, as well as disadvantages, limitations and shortcomings of size, weight, cost and complex circuitry, the present invention fulfills this long-standing need with a simplified, compact and inexpensive neutron spectrometer detector. The neutron spectrometer detector employs a thin depletion layer, silicon, solid state detector as a proton counter in an instrument that converts a distribution of neutrons to one of recoil protons. The present invention's neutron spectrometer uses computer technology to allow for greater and quicker data reduction and provides the added capability of characterizing neutron spectra by unfolding proton recoil spectra into the original neutron spectrum that produced the proton particles.
The preferred embodiment is a lightweight dodecahedron monitor for aircraft use with an arrangement of detectors, converters and absorbers housed within a sphere, further comprising a tantalum proton absorber, polyethylene hydrogenous substrate and titanium spherical chamber. The advantages of low weight, compact size, simplified operation and increased data reduction allow the present invention's neutron spectrometer to fulfill the long-standing need for measuring high-energy spectra onboard a satellite or Shuttle spacecraft, without suffering from the disadvantages, limitations and shortcomings of prior art devices. A flat dodecahedron embodiment of the neutron spectrometer with the detectors, converters and absorbers housed within a box is also described.